This invention relates to Wavelength Division Multiplexed (WDM) systems and, more particularly, to a method of and apparatus for reducing cross-phase modulation in WDM systems.
The current quest for higher transmission capacity per optical fiber has led to proposals for wavelength-division-multiplexed (WDM) transmission systems with more than 100 channels and channel spacing less than 50 GHz. In systems with uncompensated group-velocity dispersion, cross-phase modulation (CPM) can become the dominant nonlinear impairment as systems move to smaller channel spacing and larger number of channels. Recent studies of CPM have been made in the following areas:
dispersion-shifted fiber (DSF) [e.g., see article by N. Kikuchi, et al., Electron. Lett. 33, 653 (1997)],
standard single-mode fiber (SMF) [e.g., see article by T. Ogata, et al., OFC""96, vol. 1, pp. 42-44, 1996, article by L. Rapp, IEEE Photon. Tech. Lett. 9, 1592 (1997), and article by M. Shtaif and M. Eiselt, IEEE Photon. Tech. Lett. 10, 979 (1998)], and
non-zero DSF [e.g., see article by M. I. Hayee, et al., OFC""96, vol. 1, pp. 78-79,= 1996].
These studies have investigated the penalty as a function of channel power, channel spacing, and polarization. Resonances in CPM have been observed on a continuous wave (cw) probe as the sinusoidal modulation frequency of the pump was varied [e.g., see article by T. K. Chiang, et al., J. Lightwave Tech. 14, 249 (1996)].
The CPM impairment arises in a WDM system in the presence of the Kerr nonlinearity and group-velocity dispersion. The nonlinearity causes amplitude variation in signals to induce phase distortion on co-propagating channels. The group-velocity dispersion converts this phase distortion into amplitude distortion that can impact receiver sensitivity. This impairment is more commonly seen in amplitude-shift-keyed systems but can also be observed in angle-modulated systems that exhibit variation in signal amplitudes. The phase distortion occurs most strongly at the beginning of each amplifier span where the optical powers are high. In general, different wavelength channels have different group velocities, causing bits to walk through each other during transmission. This walk-off effect can reduce and virtually cancel the phase distortion induced by CPM if the pulses pass through each other sufficiently rapidly that during the course of the overlap, the power loss due to fiber attenuation is small [e.g., see article by D. Marcuse, et al., J. Lightwave Tech. 12, 885 (1994)]. This cancellation decreases as the channel spacing is reduced owing to the concurrent decrease in group-velocity difference. Cancellation of the CPM-induced phase shift will also be incomplete if pulses in adjacent channels partially overlap at the start of a span [see previously referenced Marcuse article].
There is a continuing need to more fully understand the impact of CPM in WDM systems and to devise techniques to effectively eliminate it or to cancel out its effects.
Our invention is directed to a method of and apparatus for reducing the resonances in cross-phase-modulation (CPM) impairment that occur in wavelength division-multiplexed (WDM) transmission systems having multiple amplifier spans.
Considering only a single span for the moment, deleterious phase distortions will be induced on a signal at those points of the signal that enter the fiber span coincident with large amplitude fluctuations in the copropagating signals. Large amplitude fluctuations in the interfering signals, whether occurring at clock transition points in on/off-keyed systems or arising from nonlinearity, generally will be spaced at multiples of the clock period. As the signal and interfering signals propagate in the fiber, chromatic dispersion will cause them to xe2x80x9cslidexe2x80x9d past each other.
In systems containing more than one span, the phase distortion induced on a signal in the first span can be exacerbated if the part of the signal containing that distortion enters the second span coincident with another large amplitude fluctuation in the interfering signals. This can occur if the signals slide past each other by an integral number of interfering signal bits during transmission over the first fiber span and subsequent components. That is, the induced phase distortion on a signal caused by an interferer will be enhanced when nT=xcex94xcfx84, where T is the bit period of an interfering signal and n is an integer, and xcex94xcfx84 is the difference in propagation delay for the two signals. Included in xcex94xcfx84 is all slippage that occurs between the start of one fiber span and the start of the next fiber span.
We have determined that in a two-channel, two-span WDM system, if one scans the bit-rate of one of the channels, any CPM impairment observed on the other channel will be worse at bit-rates for which the above relationship holds. In other words, a resonance will occur in the CPM penalty at these bit-rates. These CPM resonances will also be observed if the bit-rates are held constant and the channel separation is scanned. In general, the CPM impairment increases with increases in the number of interfering channels that are simultaneously in resonance with a particular signal channel. One aspect of our invention is directed to reducing such CPM resonances by mis-matching the amplifier span lengths (and/or dispersion-length product in each span) so that an integral number of bit walk-throughs do not occur between adjacent channels in successive amplifier spans.
More particularly, our invention is directed to a WDM system that includes a transmitter for sending WDM signal channels over a plurality of serially connected amplified communication spans to a receiver, each span including an optical amplifier for receiving the WDM signal channels and transmitting it over a length of fiber to an adjacent span or the receiver, and wherein at least one span comprises means for setting the clock phase difference (the time delay between bit transition times of two channels) at the fiber input between at least one pair of adjacent WDM signal channels to be different from the clock phase difference that the same channel pair has at the fiber input in at least one other span.
According to one feature, the clock phase difference of at least one channel pair at the fiber input of a plurality of spans is made adjustable and the clock phase difference at the fiber input in each span is set to be different from the clock phase difference at the fiber input in any other span. In another feature the clock phase difference of each of the channel pairs at the fiber input of a plurality of spans is made adjustable and set differently in each span.
Other techniques for reducing CPM include adding different lengths of dispersion-compensating fiber to each span, using different modulation bit rates and/or clock phase delay for each channel, and using different wavelength-selective phase delays for each channel.
If the bit-rate and channel spacing are known when designing a system, then resonances between pairs of spans can be avoided by changing the delay differences between channels.